This invention relates to a method for adjusting the shiftingxe2x80x94or transmission ratio changexe2x80x94progressiveness in a transmission device, in particular in an automatic transmission device with multiple ratios.
This invention also relates to a transmission device implementing such a method.
From WO-A-9207206, there is already known an automatic transmission in which a clutch selectively connects two rotary members of a differential gearing such as an epicyclic train, according to whether one or the other of two antagonistic forces dominates. The first force, tending to disengage the clutch, is for example a tooth reaction, more particularly an axial thrust produced by helical gear teeth mounted in an axially movable manner. The second force, tending to engage the clutch, can be produced by springs and/or by tachometric centrifugal means. When the clutch is disengaged, it is necessary to prevent rotation of a third rotary member of the differential gearing, and this can be ensured by a free wheel preventing the third member from rotating in the reverse direction.
This type of transmission is very advantageous since its basic operation does not require an external power source, nor sensors, nor a control circuit. The transmission device itself does produce the forces which will control it and these forces are at the same time a measurement of the parameters necessary for the control.
For modern transmissions which have to provide a high level of comfort and optimisation of the operation, the previously mentioned forces are advantageously completed by additional forces, produced for example by hydraulic actuators. The additional forces can serve to modify at will the speed and torque conditions under which the transmission changes ratio, or can lock the transmission in a given ratio when that is desired (PCT/FR 94/00 176).
On the other hand, it has been observed, according to the invention, that the shifting process under the effect of forces such as a centrifugal force or a tooth reaction could exhibit certain defects.
WO-A-97/08 478 proposes solutions intended to remedy certain types of defects.
A particular defect to which the present invention is relevant, is the brutality which the action of an uncontrolled force generator, such as a spring or even more so a centrifugal force generator can have, particularly in the case of the above-mentioned transmissions, when the antagonistic force disappears and suddenly releases the force generator. For example, if the antagonistic force is proportional to the transmitted torque and the latter disappears because the driver releases the accelerator pedal, the uncontrolled force generator might suddenly actuate a clutch and cause a dangerous and uncomfortable shock.
The purpose of the present invention is to provide better control of a shifting process involving actuation of at least one selective coupling means.
According to the invention, the method for adjusting progressiveness of a change from an old transmission ratio to a new transmission ratio, in a transmission device comprising:
a device for selective coupling between two rotary members;
a force generator means for urging the selective coupling means towards a predetermined one of its slipping and gripping states, corresponding to the new ratio;
an actuating means capable of urging the selective coupling means towards the other of the said states, corresponding to the old ratio;
is characterised in that the method comprises, whilst the generator means is actuating the selective coupling means towards its state corresponding to the new ratio, a step of controlling the actuating means so that it produces a measured resistant force, slowing down the transition between the old ratio and the new ratio.
By means of the actuating means, the action of the force generator means is counter-balanced in a measured manner, systematically or only when necessary, in order to prevent the force generator means from provoking a too-sudden change in the selective coupling device.
The amount of the contrary force applied by the actuating means can either be a predetermined unique amount or an amount chosen from a series of predetermined amounts, the choice being made according to a selection criterion, for example the speed of rotation which determines the centrifugal effect if the force generator means is of the centrifugal type. Such a predetermined amount preferably consists of a progressively decreasing force, which therefore progressively allows the force generator means to urge the selective coupling means towards the new coupling state.
Preferably, a physical value is detected, this value being one which is influenced by the progressive change from the old transmission ratio to the new transmission ratio, and the actuating means is controlled as a function of the result of this detection.
In the case of a transmission in which the shiftings are carried out spontaneously, that is to say without the intervention of a processing unit and, for example, are carried out according to the direction and value of the resultant of various forces such as a centrifugal force and a tooth reaction force indicating the transmitted torque, the detection of the said physical value has the function of detecting that a change of ratio is in progress in the transmission. The counter-balancing action of the actuating means is initiated, at least in certain cases, according to this detection.
Furthermore, the detection of the physical value can be used to produce a servo-control of progressiveness. It is possible, for example, to calculate the time derivative of the transmission ratio and to adjust the counter-balancing action such that this derivative remains as close as possible to a reference value. It is possible to choose references other than the derivative. For example it is possible to fix a law of evolution of the ratio over time as a reference.
When the transmission offers a number of transmission ratios which is high in comparison with the number of gear trains used, there is in general at least one shifting process which requires activation of one coupling and the release of another coupling whilst perfectly synchronising these two operations. Any imperfection in this synchronisation renders the change of ratio uncomfortable for the occupants of the vehicle and introduces stresses and/or shocks, which cause wear, in the transmission.
The selective coupling means which initiates the shifting process can in such a case be the selective coupling means which is counter-balanced as just described and this selective coupling means can be the one whose actuation varies the input speed of the transmission device in the sense corresponding to the change of ratio to be effected. When the detected physical value reaches a certain predetermined level, the actuation of the other coupling means is initiated in its turn.
As physical value which is characteristic of the evolution of the ratio change process, it is indeed advantageous to choose the input speed, or the ratio between the input speed and the output speed of the transmission or, more generally, between two speeds the ratio between which is affected by the shifting process in question.
In such a case, a progressiveness control regulating the counter-balancing provided by the actuating means as a function of the evolution of the overall transmission ratio provided by the two transmission mechanisms will allow to compensate for all of the imperfections of the ratio change process in the two mechanisms. The overall result will therefore be very satisfactory despite the complexity of the shifting process which is involved.
Another advantage in choosing as a physical value one that indicates the state of the whole of the transmission device, and not just the state of a specified subassembly such as an epicyclic train in a transmission device which comprises several of them, and in particular in choosing as a physical value the ratio between the input speed and the output speed in the transmission, is that this value indicates, according to the range of values within which it is currently varying, which change of ratio is actually occurring. For example, in a device producing four transmission ratios with only two epicyclic mechanisms in series, there are three possible changes, i.e. from the first to the second ratio, from the third to the fourth ratio and from the third to the second ratio, which comprise the engagement of the same clutch. Because of the overall detection, it is possible to distinguish which of the changes is occurring and, if necessary, to modify the control criteria accordingly. Furthermore, the overall detection makes it possible to use a same detector assembly for all of the selective coupling means for which it is desired to apply the method according to the invention.
According to a second aspect of the invention, the transmission device for a vehicle, comprising:
at least one gear train;
a selective coupling means able, by changing from an old coupling state to a new coupling state, to cause the gear train to change from an old transmission ratio to a new transmission ratio;
a force generator means capable of causing the selective coupling means to change from the old coupling state to the new coupling state;
an actuating means capable of applying to the selective coupling means an action tending to force it towards the old coupling state;
control means for controlling the actuating means;
characterised in that the control means comprise progressiveness means for causing the actuating means to apply a measured amount of force slowing down the change of the selective coupling means from the old coupling state to the new coupling state under the action of the force generator means.
In the rest of this description, following convention, a transmission ratio will be called xe2x80x9cslowxe2x80x9d when it corresponds to a high input speed with respect to the output speed. In the opposite case, the ratio is called xe2x80x9cfastxe2x80x9d.
Other features and advantages of the invention will furthermore emerge from the following description relating to non-limitative examples.